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DAΦΦΦΦNE GAMMA-RAY FACTORY
D. Alesini, S. Guiducci, C. Milardi, A. Variola, M. Zobov (INFN/LNF, Frascati, Rome, Italy);
I. Chaikovska, F. Zomer (LAL, Orsay, France)
Abstract Gamma ray sources with high flux and spectral densities
are the main requirements for new nuclear physics
experiments to be performed in several worldwide
laboratories with dedicated facilities. The paper is
focalized on a proposal of experiment on γ photons
production using Compton collisions between the
DAΦNE electron beam and a high average power laser
pulse, amplified in a Fabry-Pérot optical resonator. The
calculations show that the resulting γ beam source has
extremely interesting properties in terms of spectral
density, energy spread and γ flux comparable (and even
better) with the last generation γ sources. The energy of
the γ beam depends on the adopted laser wavelength and
can be tuned changing the energy of the electron ring. In
particular we have analyzed the case of a γ factory tunable
in the 2-9 MeV range. The main parameters of this new
facility are presented and the perturbation on the
transverse and longitudinal electron beam dynamics is
discussed. A preliminary accelerator layout to allow
experiments with the γ beam is presented with a first
design of the accelerator optics.
INTRODUCTION
Many projects worldwide, based on Compton back-
scattering between electron bunches and counter
propagating laser pulses, are aimed to generate γ photons
with high flux and high spectral densities [1-5].
For this purpose it is necessary both to increase the
number of electron-photon collisions and to control the
electron and laser beam qualities. In normal conducting
photo-injectors [6] the maximum repetition rate of the
collisions cannot exceed few kHz and also the bunch
charge cannot overcome few hundred of pC to preserve
good beam quality. On the other hand, due to the very
good beam emittance, it is possible to strongly focalize
the beam at the interaction point (IP). Then colliding
lasers with a high energy per pulse and low repetition rate
are required [3-4]. High flux and high spectral densities
can be also obtained using storage ring and high rep. rate
laser. The aim of the proposal [7] presented in this paper
is to create such a source using the DAΦNE stored
electron beam colliding with a laser beam amplified in a
Fabry-Perot Cavity (FPC) similar to the one designed and
fabricated at LAL Orsay, and used in the ATF experiment
[8].
DAΦNE is an e+e- collider operating at the energy of Φ-
resonance (1.02 GeV c.m.) [9]. Few machine parameters
[10,11], are summarized in Table I. As discussed in the
paper, the extremely high current storable in the electron
ring and the achievable beam emittance, allow reaching
excellent γ-beam qualities comparable (and even better)
with those of the new generation sources. Moreover, with
a proper choice of the machine parameters, the interval
between two consecutive collisions of an electron with a
photon can be much longer than the damping time of the
machine and the beam dynamics is completely dominated
by the dynamics of the electron ring without the laser.
Table I: DAΦNE Parameters
Energy E [MeV] 510
Machine length L [m] 97.6
Max. stored current IMAX [A] 2.5 (e- ring)
RF frequency fRF [MHz] 368.67
Max RF voltage VRF_MAX[kV] 250
Harmonic number hN 120
Min.bunch spacing TB[ns] 2.7 (=1/fRF)
Hor. emittance εx [mm mrad] 0.250
Coupling coupl=εy/εx [%] <0.5
Bunch length σt [ps] 40-60
Energy spread ∆E/E [%] 0.04-0.06
Long. Damp. time τdamp [ms] 17
COLLISION SCHEME AND RESULTS
Compton sources can be considered as electron-photon
colliders. For a generic γ-source there are four important
quantities that characterize the source: the total number of
scattered photons per second over the 4π solid angle, the
rms source bandwidth (BW), the number of photons per
second in the bandwidth and the spectral density. These
quantities can be calculated with simple formulas that can
be found in [4,7,12-15].
A simple sketch of the interaction region is given in Fig.
1.
Figure 1: Simple sketch of the interaction region.
Since, in the DAΦNE ring, the coupling can be small
(typically <1%) we have considered, at the IP, the case of
a flat electron beam and a round laser beam with equal
vertical dimensions. It is easy to demonstrate [7] that, to
have an equal contribution of the electron beam emittance
to the γ final energy spread in both horizontal and vertical
planes, the ratio of the β-functions βy/βx at the IP has to
be equal to the machine coupling. The results of the
calculations are given in Fig. 2-3 where few important γ-
source parameters have been plotted as a function of the
6th International Particle Accelerator Conference IPAC2015, Richmond, VA, USA JACoW PublishingISBN: 978-3-95450-168-7 doi:10.18429/JACoW-IPAC2015-TUPWA055
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2: Photon Sources and Electron AcceleratorsA23 - Accelerators and Storage Rings, Other
laser spot size. For the rms bandwidth calculation we
have assumed that the main contribution to the energy
spread is due to the collection angle and e- beam
parameters. We have considered the case of 1.5 A in 60
equally spaced bunches that is compatible with the best
performances of the machine in the last SIDDARTHA
run [10] and 2.5% of coupling. The emittance has been
considered to be 40% of its present value. As shown in
[7,16] this value can be achieved in DAΦNE with a
proper optics design. For the laser we have considered the
case of λL=1µm with a collision angle of 8 degrees and a
collisions repetition frequency of 184 MHz (fRF/2). All
these parameters are compatible with those of the FPC
used in ATF [17]. The results show the extremely good
quality of the source both in term of flux and in term of
SPD. The best results have been summarized in Table II
for two cases of 1% and 2.5% of coupling.
Table II: DAΦNE γ Source Parameters
Parameter Case 1 Case 2
e- stored current [A] 1.5
number of bunches 60
e- emittance [mm*mrad] 0.100
coupling @ IP [%] 2.5 1
e-/laser pulse length [ps] 50/20
e- σy (=laser) @ IP [µm] 40.0 30.0
e- hor. dim. @ IP [µm] 1600.0 3000.0
e- vert. βy @ IP [m] 0.6 0.9
e- hor. βx @ IP [m] 25.6 90
laser stored power [kW] 36.8
Laser energy per pulse [µJ] 200.0
laser wavelength [µm] 1.0
Max. γ energy [MeV] 4.94
total γ flux [ph/s] 0.96⋅1012
0.75⋅1012
γ energy spread [%] 0.57 0.21
γ flux in the BW [ph/s] 5.8⋅109
1.7 ⋅109
SPD [ph/s/eV] 81533 64158
Max. collecting angle [µrad] 63 39
Figure 2: γ flux in the BW (top) and rms BW (bottom).
The energy of the γ beam can be changed by varying the
energy of the laser, the energy of the electron beam or in
another configuration the incidence angle. In order to
avoid a strong reduction of the beam lifetime, it is
important to maintain the maximum energy of the γ
photons within the energy acceptance of the ring. The
maximum energy of the γ beam as a function of the
electron beam energy for λL equal to 10, 1 and 0.5 µm are
given in Fig. 4. In the same figure it is also reported the
required energy acceptance. The energy up to 700 MeV
can be, in principle, reached in the DAΦNE electron ring
by increasing the magnetic elements strength. Typical
DAΦNE energy acceptances are below 1.5% (unless
considering very high RF voltages and/or very low
momentum compaction factors). As clear from Fig. 4, this
fact forces the wavelength of the collision laser to
mL
µλ 1≥ . The resulting energy of the γ photons can be
then tuned reasonably in the range 2-9 MeV.
Figure 3: Spectral density.
Figure 4: Maximum energy of the γ beam (top); required
energy acceptance (bottom).
ELECTRON BEAM DYNAMICS
The electrons colliding with photons emit γ rays with
energies proportional to the square of the electron energy
and with a typical average spectrum [18-19]. This causes
a radiation damping but also a quantum excitation of the
beam energy since the γs are stochastically emitted in
quantum. Moreover, the γ rays are emitted at different
angles and there is a typical correlation between the
energy and the angle [18-19]. The final transverse
emittance and energy spread are due to the balance
between these effects and those due to the synchrotron
light emission in the ring [21]. Looking at the machine
parameters given in Table II it is straightforward to
recognize that the average interval between two
6th International Particle Accelerator Conference IPAC2015, Richmond, VA, USA JACoW PublishingISBN: 978-3-95450-168-7 doi:10.18429/JACoW-IPAC2015-TUPWA055
2: Photon Sources and Electron AcceleratorsA23 - Accelerators and Storage Rings, Other
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consecutive collisions of an electron with a photon is
much longer than the damping time of the machine and,
as a consequence, the beam dynamics is completely
dominated by the dynamics of the electron ring without
the laser. This intuitive conclusion is more rigorously
confirmed by following a theoretical approach or Monte
Carlo simulations that we performed using the tracking
code CAIN [7,20]. This code also validated the
theoretical calculations. Some results are given in Fig. 5-6
for the parameters reported in Table II-case 1 [7].
Figure 5: γ spectrum after collimation collimation.
Figure 6: γ rays spatial distribution after collimation.
POSSIBLE IMPLEMENTATION
A sketch of a possible collider implementation is shown
in Fig. 7 with the experimental hall located near the
present Cryogenics hall. The FPC can be placed in the
first interaction region. As pointed out in the previous
sections, the calculations we have done are referred to the
case of the FPC already realized by the LAL Orsay
Laboratory and successfully tested [17]. The picture of
the cavity is given in Fig. 8. Few modifications have to be
implemented but there are enough margins for them [7].
Figure 7: Possible implementation of the γ factory.
A preliminary lattice for the DAΦNE γ factory has been
also elaborated [16]. The DAΦNE lattice has enough
flexibility to vary the emittance in the range between 0.28
and 0.045 mm⋅mrad. In this approach the DAΦNE layout
has been modified to insert a dogleg section necessary for
the extraction of the γ beam. The easiest way to achieve
this, is to install two magnets in the interaction region
straight section with an angle of 5 degrees and to modify
the bending angle of the nearest dipoles. This layout
allows to easily extract the γ beam line from the ring
vacuum chamber. As Interaction Region for the γ factory,
it is possible to use either the center of the straight section
(between the dogleg dipoles) or the 2.5 m long straight
section adjacent to the “Short” arc. The optical functions
in half of the ring are shown in Fig. 9, the other half ring
being symmetric.
Figure 8: Pictures of the LAL Orsay FPC.
Figure 9: Lattice optical functions for half ring.
CONCLUSIONS
Preliminary studies show that tunable γ photons beams
can be generated in DAΦNE using Compton collisions.
The photons energy can be tuned in the 2-9 MeV range.
The calculations have shown that the resulting γ beam
source has extremely competitive properties in terms of
spectral density, energy spread and γ flux. The main
parameters of this new facility are feasible. For this
experiment there is the possibility to use (with few
feasible modifications) the FPC designed and fabricated
by the LAL Orsay Laboratory and already tested at ATF.
Besides, preliminary DAΦNE layout and low emittance
optics have been elaborated.
ACKNOWLEDGEMENTS
We would like to thank L. Serafini, A. Ghigo and M.
Ferrario for their helpful discussions and suggestions.
6th International Particle Accelerator Conference IPAC2015, Richmond, VA, USA JACoW PublishingISBN: 978-3-95450-168-7 doi:10.18429/JACoW-IPAC2015-TUPWA055
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2: Photon Sources and Electron AcceleratorsA23 - Accelerators and Storage Rings, Other
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6th International Particle Accelerator Conference IPAC2015, Richmond, VA, USA JACoW PublishingISBN: 978-3-95450-168-7 doi:10.18429/JACoW-IPAC2015-TUPWA055
2: Photon Sources and Electron AcceleratorsA23 - Accelerators and Storage Rings, Other
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